Wear resistant steel

ABSTRACT

1. A QUENCHED AND TEMPERED FERROUS DIE HAVING IMPROVED WEAR RESISTANCE, AS EXEMPLIFIED BY A HIGH RESISTANCE TO SCORING, CHECKING AND PICK-UP OF THE STEEL SHEET AND STRIP FORMED WITHIN SAID DIE, WHERE SAID IMPROVED WEAR RESISTANCE IS IMPARTED BY THE PRESENCE OF NITRIDES OF TITANIUM, COLUMBIUM AND/OR ZIRCONIUM, SAID DIE COMPRISING A QUENCHED AND TEMPERED FERROUS ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT, CARBON BETWEEN ABOUT 0.40 TO 1.30%, MANGANESE BETWEEN ABOUT 0.10 TO 3.00%, NITROGEN BETWEEN ABOUT 0.007 TO 0.050%, ABOUT 0.02 TO 0.25% TOTAL OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF TITANIUM, COLUMBIUM AND ZIRCONIUM, WITH THE BALANCE ESSENTIALLY IRON.

Nov. 5, 1974 MELLOY ETAL 3,846,184

WEAR RESISTANT STEEL Filled July 10, 1972 Fly.

United States Patent 3,846,184 WEAR RESISTANT STEEL George F. Melloy, John Y. Riedel, and Paul P. Podgursky, Bethlehem, Pa., assignors to Bethlehem Steel Corporation Continuation-impart of application Ser. No. 753,167, Aug. 6, 1968, which is a continuation-in-part of application Ser. No. 628,887, Apr. 6, 1967, now abandoned. This application July 10, 1972, Ser. No. 270,389

Int. Cl. C04b 35/70; C22c 39/14 US. Cl. 148-36 3 Claims ABSTRACT OF THE DISCLOSURE An improved ferrous alloy characterized by a high degree of wear resistance in the as-rolled, and quenched and tempered conditions, which alloy has particular utility as a grinding mill rod, and steel sheet and strip forming die, respectively. The Wear resistance is imparted by the presence of nitrides of titanium, columbium and/or zirconium. Said alloy consists essentially of 0.020.25% total weight percent of one or more elements selected from the group consisting of titanium, columbium and zirconium, 0.007-0.050% by weight nitrogen, with the balance essentially iron, said alloy being further characterized by a microstructure consisting essentially of a well dispersed mixture of nitrides and carbides in a pearlitic matrix after air cooling from a hot-working temperature, and when quenched and tempered, of a well dispersed mixture of nitrides and carbides, in a martensite matrix.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation of Ser. No. 753,167, filed Aug. 6, 1968, which in turn is a continuation-in-part of Serial No. 628,887, filed Apr. 6, 1967, now abandoned.

BACKGROUND OF THE INVENTION The problems associated with improving the wear resistance of steels are well known, and it is generally accepted that wear resistance of steels is directly proportional to carbon content and hardness. It is also known that alloy additions and/or heat treatments are very effective means for improving wear resistance.

The rod mills used by the mining industry to reduce the particle size of ores so as to facilitate the separation of impurities therefrom consume large quantities of round steel bars as a result of wear. Rod mill rods are in direct rolling contact with the ore which is being ground and resistance to abrasive wear is a desirable quality. From the economic standpoint, the preferred material for these rods at the present time is as-rolled carbon steel, e.g. C1090. Alloy additions to, and/or heat treatments of, this basic grade have been unsuccessful in the past, since the degree of improvement in wear resistance has been insufliicient to oifset the cost of the alloys and/or heat treatment.

Cold-work die steels are widely used in forming and ironing operations on steel sheet and strip. Such die steels are generally carbon steels, containing 0.80 to 1.30 wt. percent carbon, and are quenched and tempered to a surface hardness of from 50 to 67 Rockwell C. A Rockwell C hardness of 50 is usually considered the minimum useful hardness for carbon steel dies of this carbon content, while a Rockwell C hardness of 67 is the maximum hardness attainable. Steel dies formed of these steels should have a high resistance to scoring, checking and pick-up of the reel being formed. The desired resistance to wear is somewhat dififerent for die steels than for rod mill rods, as any wear in the surface of a die must be ice immediately corrected, while the surface of rod mill rods is relatively unimportant, the entire rod being eventually consumed by the rod mill.

It is an object of this invention to provide a steel having improved wear resistance in both the as-rolled condition (e.g., for use as rod mill rods) and the quenched and tempered condition (e.g., for use as dies).

SUMMARY OF THE INVENTION It has been discovered that nitrides of the elements titanium, columbium and zirconium are very effective in improving the wear resistance of steel, and that steels which contain such nitrides, together with suflicient carbon to essentially eliminate the formation of ferrite during air cooling from the hot-working temperature, are characterized by improved wear resistance in both the asrolled and quenched and tempered conditions. Furthermore, this improvement is achieved at a very low cost.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are photomicrographs illustrating the microstructure of the steels of this invention at 500 magnifications.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Broadly, the steels of this invention consist essentially of:

TABLE 1 Wt. percent Carbon 0.40 to 1.30 Manganese 0.10 to 3.00 Nitride-Forming Element The balance iron. 0.007 to 0.050

*0.02 to 0.25 total wt. percent of one or more nitride-forming elements from the group consisting of titanium, columbium and zirconium.

By balance iron we do not wish to exclude residual impurities and incidental elements which may be present in amounts which do not substantially detract from the properties of the steels. For example, the steels are killed, usually by the addition of silicon and aluminum thereto, and may contain 0.10 to 1.00 wt, percent silicon and up to about 0.10 wt. percent aluminum.

A minimum of 0.40 wt. percent carbon has been found essential to obtain a microstructure which, after air cooling from a hot working temperature, is free from ferrite and consists essentially of carbides in a pearlitic matrix. Such a microstructure is fundamentally important for good wear resistance. Carbon above about 1.30 wt. percent adds little to wear resistance, and detracts from toughness and workability. A minimum of about 0.10 wt. percent manganese is necessary for working, e.g., rolling, the steel, while no benefits are obtained by increasing the manganese above about 3.00 wt. percent.

The improved wear-resistant properties of the steels of the invention are primarily due to the presence therein of nitrides of titanium, columbian and/or zirconium. Titanium nitrides are the most wear-resistant of this group, and we therefore prefer to utilize titanium as a nitrideformer. Improvements in wear resistance are apparent when the steels contain a minimum of about 0.007 wt. percent nitrogen and about 0.02 wt. percent titanium, while little improvement results from nitrogen above 0.050 wt. percent and titanium above 0.25 wt. percent. In addition, the low solubility of nitrogen makes it diflicult to produce steel containing more than about 0.025 wt. percent. nitrogen.

Within the above broad range, there are both narrow and preferred ranges for the subject steels, dependin upon whether they are to be used in the as-rollecl condition (e.g., as rod mill rods) or in the quenched and tempered condition (e.g., as dies). These ranges are as follows:

TABLE 1A Weight percent Narrow Preferred A. Rod mill rod steel:

Carbon 0. 70-1. 0. 85-0. 98 Manganese- 0. 30-1. 30 0. 60-0. 00 Silioon 0. -0. 50 0. 15-0. 30 Ti, Cb and/or Zr 0.02-0.25 0. 08-0. 15 Nitrogen 0 007-0. 025 0. 010-0. 020

Balance iron. B. Die steel:

Carbon 0. 80-1. 30 1. 00-1.10 Manganese 0. 10-0. 40 0. -0. 35 Silicon 0. 10-0. 40 0. 20-0. 30 Ti, Cb and/or Z 0. 02-0. 0. 08-015 Nitrogen 0. 007-0. 025 0. 010-0. 020

Balance iron.

The major differences between the narrow ranges of the rod mill rod steel and the die steel lie in their carbon and manganese contents. The rod steel contains a minimum of 0.70 wt. percent carbon so that a large number of carbides are present. However, the maximum carbon should be limited to 1.10 wt. percent, as higher amounts embrittle the steel and increase the probability of breakage during utilization in a rod mill. The manganese range of the rod steel has been chosen so that the steel can be easily worked, the manganese apparently having little effect on the final properties of the steel.

The die steel has somewhat higher carbon and lower manganese than the rod steel. The reason for this is that the die steel is utilized in the quenched and tempered condition. A minimum of 0.80 wt. percent carbon is necessary to obtain the minimum desired surface hardness, while carbon above about 1.30 wt. percent results in a steel containing excessive cementite. The manganese should be limited to about 0.40 wt. percent, as higher amounts increase the susceptibility of the steel to crackmg.

The die steel may contain about 0.15 to 0.25 wt. percent vanadium to insure a fine grain size.

Rod Steel The compositions of a representative open hearth heat of C1090 steel and six heats representative of the rod steels of this invention are shown in Table 2 below:

TABLE 2 0 Mn P S Si Ti N As-rolled C1090--. 96 77 012 027 27 Nil Residual 1 .81 1.05 .02 .017 .35 .06 .014 98 02 018 31 07 018 1. 04 02 014 3 09 016 1. 12 02 018 39 09 020 1. 02 02 016 34 l1 018 99 02 020 34 16 024 Residual nitrogen content of open hearth and basic oxygen steel varies from about 0.003 to 0.006 wt. percent, and averages about 0.004 to 0.005 wt. percent.

Samples of the steels of Table 2 were hot rolled to 1" diameter bars at temperatures between 2200 F. and 1900 F. in the normal manner and air cooled to room temperature.

FIG. 1 illustrates the distribution of titanium nitrides observed on an unetched polished surface of a sample from the 1" diameter bar of Heat No. 5 and is representative of the microstructure of steels of this invention after air cooling from a hot-working temperature.

FIG. 2 illustrates the distribution of titanium nitrides on the surface of another sample from the same heat after etching with 2% Nital so as to show the lamellar pearlitic background. FIG. 2 shows the nitrides in the steel of this invention are well dispersed throughout the pearlitic matrix in the as-rolled condition.

The as-rolled hardness of these 1" diameter bars is shown in Table 3 together with the results of an empirical Wear test consisting of measuring the percentage reduction in the diameter of an abrasive cut-off wheel used to make ten cuts through each of these 1" diameter bars.

As can be seen from Table 3, the as-rolled titaniumnitrogen rod steels of this invention reduced the diameter of an abrasive cut-off wheel to a greater extent than asrolled C1090 steel. Although these empirical test results fail to provide any data to estimate the wear resistance under different service conditions, they show that the combination of titanium and nitrogen in the amounts of this invention improves the wear resistance of ferrite-free as-rolled rod steels under these particular test conditions.

The results of another comparative test of the wear resistance of the rod steel of this invention with as-rolled C1090 steel under conditions similar to those encountered in the production grinding of taconite ore are summarized in Table 4.

TABLE 4 Weight loss, 0 Mn P S St Ti N2 lbs/ton As rolled C1090..-" .88 .92 .009 .007 .25 Nil .00355 Ti-N steel .92 .97 .010 .008 .19 .10 .009 .00323 Residual.

The bars used in this test were as hot-rolled to 4%" diameter and indicated that the rod steel of this invention may provide about a 9% reduction in the weight loss en- Nitrogen was added to this steel in the form of nitrided' ferromanganese manganese) which was placed on the bottom of the ladle into which the heat of steel was tapped. The titanium (approximately pure) was added to the ladle after about half the metal was in the ladle. The ladle analysis showed 0.10 wt. percent titanium and 0.010 wt. percent nitrogen. Pouring the steel into hottopped ingot molds was started approximately two minutes after completing the tap. The best recovery of titanium and nitrogen was obtained by teeming the heat into the molds as soon as possible after tapping the heat. After solidification the ingots were heated to approximately 2300 F., hot rolled to 4 /8" diameter bars in the usual manner, and air cooled to room temperature.

Die Steel A heat of steel having the following ladle analysis was produced by conventional procedures.

Although the manganese content of this steel was considerably higher than desirable for use as a die steel, the steel was hot rolled into a bar, air cooled to room temperature, rough machined and ground into a die, water quenched, tempered to a hardness of 62 Rockwell C, and finish ground.

The test die so produced was installed and was run day, i.e., with no lubricant, for four days, producing 64,778 parts before major repair, e.g., welding of scoring defects, of the die was required. The normal production capacity of dies of similar steel grades, but not containing titanium and nitrogen within the ranges of the invention, is 2,400 parts before major repair is required.

As a specific example of the method of producing the die steel of this invention, a 25 ton of steel was produced in a basic electric-arc furnace. The ladle analysis of the steel was as follows:

The steel was poured into ingot molds and hot-topped. Following solidification, the ingots were heated to 2070 F., hot rolled into billets in the usual manner, and air cooled to room temperature.

6 All compositions disclosed herein are in terms of wt.- percent.

We claim:

1. A quenched and tempered ferrous die having improved wear resistance, as exemplified by a high resistance to scoring, checking and pick-up of the steel sheet and strip formed Within said die, where said improved wear resistance is imparted by the presence of nitrides of titanium, columbium and/ or zirconium, said die comprising a quenched and tempered ferrous alloy consisting essentially of, by weight, carbon between about 0.40 to 1.30%, manganese between about 0.10 to 3.00%, nitrogen between about 0.007 to 0.050%, about 0.02 to .25% total of at least one element selected from the group consisting of titanium, columbium and zirconium, with the balance essentially iron.

2. The ferrous die claimed in claim 1 wherein the carbon is present in an amount between about 0.80 to 1.30%, manganese between about 0.10 to 0.40%, a maximum nitrogen of about 0.025%, and including silicon between about 0.10 to 0.40%.

3. The ferrous die claimed in claim 2 wherein the carbon is present in an amount between about 1.00 and 1.10%, manganese between about 0.20 to 0.35%, nitrogen betwen about 0.010 to 0.020%, a total of 0.08 to 0.15% for said titanium, columbium and/0r zirconium, and silicon between about 0.20 to 0.30%.

References Cited UNITED STATES PATENTS 2,343,956 3/1944 Crafts l23 R 2,155,348 4/1939 Grossmann 75l23 M 2,987,429 6/ 1961 Smith 75l23 B 2,527,731 10/1950 Ilacgug 75l23 B 2,603,562 7/l952 Rapatz 75l23 B 2,121,056 6/1938 Smith 75l23 B HYLAND BIZOT, Primary Examiner US. Cl. X.R.

75l23 B, 123 H, 123 J 

1. A QUENCHED AND TEMPERED FERROUS DIE HAVING IMPROVED WEAR RESISTANCE, AS EXEMPLIFIED BY A HIGH RESISTANCE TO SCORING, CHECKING AND PICK-UP OF THE STEEL SHEET AND STRIP FORMED WITHIN SAID DIE, WHERE SAID IMPROVED WEAR RESISTANCE IS IMPARTED BY THE PRESENCE OF NITRIDES OF TITANIUM, COLUMBIUM AND/OR ZIRCONIUM, SAID DIE COMPRISING A QUENCHED AND TEMPERED FERROUS ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT, CARBON BETWEEN ABOUT 0.40 TO 1.30%, MANGANESE BETWEEN ABOUT 0.10 TO 3.00%, NITROGEN BETWEEN ABOUT 0.007 TO 0.050%, ABOUT 0.02 TO 0.25% TOTAL OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF TITANIUM, COLUMBIUM AND ZIRCONIUM, WITH THE BALANCE ESSENTIALLY IRON. 